The present invention relates to an image radiographing system that employs radiation for radiographing.
Medical images obtained through radiographing by the use of radiation have been widely used as an image for diagnoses. It is important to control properly a radiation dose to be irradiated, in radiographing, for avoiding excessive radiation exposure. Further, a radiation dose and image quality are closely related to each other, and if a radiation dose is not proper, graininess of an image is worsened and diagnostic performance is lowered. It is therefore important to control a radiation dose properly, also for securing diagnostic performance of images. Therefore, on a radiographing apparatus that performs radiographing, there is mounted an automatic exposure controller (AEC) so that a radiation dose may be controlled to be appropriate.
The automatic exposure controller is equipped with a sensor to detect a radiation dose transmitted through a subject, and a period of time for radiation irradiation is controlled based on the radiation dose detected by this sensor. When the diagnosing object is a breast, it is important to irradiate radiation in an optimum amount of dose to mammary gland tissue, because most breast cancers are caused from mammary gland tissue. There is proposed a technology to show where the sensor is located on a breast by projecting a sensor position on the breast before irradiation of X-ray, so that the position of the sensor that detects a radiation dose may agree with a portion where the mammary gland density is highest (for example, see Patent Document 1)
However, some of radiation images are hard to read because of characteristics of the images or characteristics of the subject itself. For example, in the case of examination radiographing, mammary gland tissue and fat tissue are intermingled on a breast. Breast images are classified into the following four types of breast (hereinafter referred to as a breast type), depending on a rate of mammary gland tissue within the breast and on the state of distribution of mammary gland tissue.
(1) Fatty Type
This is a type wherein a breast area is almost perfectly replaced by fat. Since a fatty area indicates high density on a film image, and an affected area (cluster of micro-calcifications and a tumor) indicates low density, if the affected area is included in an area of radiographing, it is easy to detect the affected area.
(2) Mammary Gland Scattered Type
This is a type wherein mammary gland substances are scattered in a breast area that is replaced by fat. It is relatively easy to detect an affected area.
(3) Heterogeneous High Density Type
This is a type wherein a plurality of fat tissue are scattered in mammary gland substances, and heterogeneous density is indicated. Since a mammary gland tissue indicates low density on an image compared with fat, there is a possibility that an affected area is veiled by the mammary gland tissue.
(4) High Density Type
Fats are hardly intermingled in mammary gland substances, and a detecting rate for affected areas is low.
In a breast image classified to be high density type among the aforesaid various breast types, sufficient contrast cannot be obtained, because mammary gland tissues are distributed uniformly, and an area of the same density extends in a mammary gland area. Therefore, it has been difficult to discover an affected area such as a microscopic calcified cluster wherein microscopic low density dots are gathered to exist in a mammary gland area. In contrast to this, in another breast type such as the mammary gland scattered type, contrast is easily indicated in a mammary gland area, and it is relatively easy to detect an affected area, because high density fatty areas are intermingled in the mammary gland area.
As stated above, characteristics of images are different from each other depending on the structure of tissues having an individual difference for each subject such as a mammary gland area. Therefore, if the same image processing condition is used for image processing for breast images obtained through radiographing, it is impossible to acquire the optimum image, which has been a problem.
[Patent Document 1] Japanese Patent Publication Open to Public Inspection No. 8-80295